Magnetic pickup device



7, 1945. J. c. ADAMS 2,381,097

MAGNETIC PICK-UP DEVICE Filed Oct. 29, 1942 i A s .42; I

L Juvcnror J as 21111 C Adams Gttorncg Patented Aug. 7, 1945 mounrrc rrcxur Davies Joseph 0. Adana-East Orange, N.'J., assignor to Thomas A. Edison, N. 1., a corporation Incorporated, West Orange.

! New Jersey Application October 29, 1842, Serial No. 483,798

Claims.

This invention relates to a magnetic pick-up device of the type adapted for response to a magnetic field, and has especial relation to magnetic pick-up devices adapted for picking up signal energy from telephone systems as for the purpose of coupling a phonographic recorder or .loudspeaking system to a telephone.

It is an object of my invention to provide a magnetic pick-up device having an improved emciency.

It is another object to provide an improved magnetic pick-up device which is selectively responsive to magnetic fields oi! different divergence-that is to fields originating from sources at difierent distances.

It is another object to provide a selective magnetic pick-up unit the parts of which do not re quire to be individually adjusted 0r positioned relative to the magnetic fields which are to be selected between.

It is another object to provide a selective magnetic pick-up device in a unitary form which does not require a critical positioning of the unit relative to the magnetic fields which are to be selected between.

It is a further object to provide a magnetic pick-up unit which is effective, when placed adjacent to a telephone box containing the standard telephone induction coil, to pick up signal energy ircm the telephone system with an improved signal to noise ratio.

It is a still further object to provide a magnetic pick-up unit which isespecially adapted for conveniently picking up telephonic voice signals from hand combination telephone sets.

Magnetic pick-up devices for inductively coupling electrical apparatus such as a phonographic recorder or loudspeaking system to a telephone system are well known in the art. Ordinarily these pick-up devices are placed on or adjacent to the telephone box where they will be linked by the stray flux oi the standard induction coil of the telephone equipment. The telephone box may be either on the wall or within the base portion of a desk set such as is the case with hand combination set commonly used. In either case though the magnetic shielding provided by the box container prevents eiilcient coupling of the pick-up device with the telephone system stand, as a result, a low level of signal pick-up is Other and allied objects of my invention will more iully appear from the following description and the appended claims.

In. the description of my invention reference is had to the accompanying drawing of which:

Figure l is a side view, partly broken away, 01' a hand combination telephone set showing the pick-up device oi my invention associated therewith for picking up signals from the telephone induction coil;

Figure 2 is a perspective view showing a proper positioning of the pick-up coils 01' my device rela tive to a telephone induction coil;

Figure 3 is a top plan view, with parts broken away. of the pick-up device according to my invention;

Figure 4 is an enlarged fractional sectional view taken on the linu 4-4 of Figure 3; and

Figure 5 is a schematic circuit diagram oi. my pick-up device and oi a typical load therefor,

obtained. In fact, often stray fields or electrical" machinery or apparatus in the vicinity are comarable in strength to the available signal held from the telephone system, and noise pick-up is a serious problem. To subdue however the noise pick-up, there has been provided a magnetic pickup device 0! a dual i'orm comprising two electrically connected but separately positionable pickup coils. One of these coils is placed in the signal field from the telephone system but the other is placed substantially out of that field and so oriented with respect to the noise field as to new tralize the noise flux-currents picked up by the other coil.

Magnetic telephone pick-up devices as heretotore provided have however not been very satisfactory in that they have been wanting for a better signal to noise ratio of pick-up-this being especially true when picking up from a closed core type of telephone induction coil-and have been large and bulky, making them difiicult to mount. Moreover, it'has been a disadvantage of those of the dual form that they have required a separate and critical relative positioning of the pick-up coils. I have found however that these deficiencies can be largely overcome by the use of long slender solenoidal type coils. These coils are wound on magnetic cores which have their several dimensions in cross section substantially equal, or varying at most by no more than a ratio of 2:1. Preferably, I use magnetic cores having;

sectional area. as at least a large ratio of length to square root of cross of the order of 15 but preferably more, and apply windings'on these cores having a radial thickness not substantially greater, but preterablyless, than the dimensions in cross section of the cor th cells having a ratio or length to outside diameter which is at so t at I am enabl d to obtain least 01 the order 01 ,5. With such coils. I find an improved coeiiicient oi coupling with the induction coil, whether thatcoilbeinahandcombinationsetorwali.

box, or oi the substantially closed core type, and thatlamespeciallyenabledtorealiseasmall pick-up device -in a unitary iorm which has a substantially improved pick-up ratio oi signal to noise without need ior separate positioning oi its several coils or of critical positioning oi the unit as a whole.

In a preierred iorm oi my invention shown in Figures 3 and s, which is oi the dual iorm abovenoted, there are provided two coils I and la. These coils are made substantially identical, within the tolerances oi ordinary manuiacturing practice, and are mounted on laminated cores I and' Each coil 2a made for example oi silicon steel. is covered with a iold oi electrostatic shielding I, as oi tin ioil, but the overlapping ends oi the toil are separated by an intervening sheath oi insulation-l, as is indicated in" Figure 4, this being done so as not to place a short circuit around the coils. Purely by way oi. example. the pick-up device will be considered to work into an unbal-' anced load circuit, as into the input of an electrical amplifier, in which case connection maybe made to the load by a shielded single conductor lead I oi which the shield Q is utilized as a ground return connection, this shield 0" being connected by a lead I to .one oi the coils. By suitable connections not herein necessary to show, the shield I is also connected to the coil shields Iandtotheccresl andla. 'Ihecoils l and is are fixedly held in substantially axially parallel relation by a non-magnetic cross-piece II and respective clamps II, the clamps being insulated from the coils by insulation wrapping: II and arranged so as not to shcrt-circuitthe coils. This coil arrangement is mounted in a thin-walled non-magnetic container it, made as oi plastic, and is held therein by a screw ll, the container having any suitably removable cover piece ii and side opening it through which the shielded conductor I passes. The laminated cores 2 and la terminate respectivelyinto shoes i1 and ila at thecoilends,andtheseshoesaredisposedradially oi the coils in parallel relation to one another andextendatleastasiarastotheperipheryoi the coils for contact with the cover oi the container ll.

The pick-up'ccils i and is are to oppose one another as magnetic pick-up elements, and may be connected either in series or in parallel arrangement. I preier, however, to use the series arrangement and do accordingly interconnect the adjacent ends oi the coils by a shielded conductor I, the shield oi which will be understood to be connected to be electrically ccmmonwith the other :shielding elements oi the pick-up device.

' When two identical and axially parallel coils are so serially connecteiL. their pick-up irom a given magnetic field will be such that their respectively induced currents will oppose or-buch one another.

'Ihis serial connection and spatially parallel relationoithecoils,bywhichtbeycpposeonsanother as magnetic pick-up elements. I herein-term "a series-opposing one."

Fcrpickinglmsignalsiromaninducticncoil is, the pick-up device may be positioned so that theaxesoithecoilsareparalleltotheaxis'cithe induction coil with one oi the pick-up coils ate (with which the magnetic core oi the induction coil is provided as is hereinaiter explained) with one oithe ccilsbeing again spaced at a minimum possible distance iron the induction coil. This induction coil is a standard element in telephone equipment and may be provided with or without a substantially closed magnetic core. In the present illustrative example. a substantially closed core type of coil is shown having the laminated magnetic core I! provided with air gaps II abovementioned at the opposite ends thereoi, these air gaps being provided to prevent saturation oi the core irom the D. C. current oi the telephone line. Because there is a magnetic potential drop across the coils it necessarily has a stray fiux radiation although the. radiation is less as the reluctance core induction coil, it is highly important that the maximum possible coeiilcient oi coupling be made, that is, that there be obtained a maximum linkage oi the pick-up coil with the available stray flux oi the induction coil.

To obtain such maximum linkage, the core oi the pick-up coil is aligned with the available stray iield oi the induction coil as by orienting theaxis oi the pick-up coil in a plane substantially parallel to that oi the induction coil. According to my invention I am enabled to obtain a high percentage oi fiux interlinkage with the pick-up coil by the use oilong slender coils having magnetic cores, as this permits the magnetic cone oi the coil to be placed at a minimum spacing item the induction coil, and also through the length oi that coil, where it will provide a low reluctance path for the' available stray aux Moreover, the core tire length oi core oi the pick-up coil to efiectivelv increase the cceiiicient oi coupling and efiiciency oi pick-up.

I find that the use oi long slender pick-up coils is particularly eiiective ior obtaining a pick-up device that is highly selective as to fields originating from sources near and iar, or to fields oi diiierent divergence, it being understood that fields near their source have a high divergence-that is, a considerable digression irom parallelismand those at a distance irom their source have a low divergence. This selectivity arises item the iact that'a long magnetic core exercises a high directive influence over the flux in its vicinity, since the fiux, in seeking a path oi low reluctance, is "taken in" by the core and led along the length thereof, the effect being to bunch and straighten out the fiux lines in the vicinity oi'the coil. Both oi the pick-up coils will exercise this directive inminimum possible distance irom the induction coil.asisindicatedinl"igurc2: r tpreierablr. however, the pick-up device may be oriented so that the axes ci-the'pick-up coils are parallel to acenterlinerunningthroughtheairgapsll iiuence alike and will tend'to. be interlinked by about the same amount oi fiux unless the magnetic field has a large divergence. Normally, noise pickup is irom fields oi small divergence since the sourceoithencise,suchasamotor,willbeata great distance irom the pick-up device compared to that oi the pick-up device irom the induction coil oi the'teleph'one instrument. The pick-up ecilswillthustendtopickupapproximatelythe sameamotmtoi noise. lloreoveatheywilltend todothiswithinarange oirelative positioning. aswellssoipositioningciboththecoilsaiihs relativetothenoisefle tbvvirtusoithslsns directive influence which the coils exercise over the flux in their vicinity. Accordingly, my pickup device does not require that the coils thereof be separately adjusted nor that the over-all device be critically located in order to effect a low pick-up of noise current originating from sources at a distance,

For maximum pick-up from the telephone system, the pick-up device is so oriented as to obtain the largest possible differential of coupling of its respective coils with the telephone induction coil. For instance, one coil may be placed directly below the induction coil, or be shifted from that position in the direction which will increase the spacing of both the coils from the induction coil, a suitable positioning of the pick-up coils relative to the induction coil being, for instance, shown in Figure 2.

It will be understood a substantial spacing must be had between the pick-up coils I and la in order that a substantial differential in coupling of the coils with any nearby flux source may be had. Yet the distance of spacing must not be too great, to result in any substantial difference in coupling of the coils with a relatively distant flux source. As a typical case, the spacing between coils is about half of their length. For example, the coils are preferably made about 3 long and spaced about 1 apart, and are provided with an outside diameter of about tic". As for the cores, they are preferably made about square in cross section with a dimension of about at each side, and the shoes thereof are each made about long.

Because of the very low coefficient of coupling of the pick-up device with the telephone induction coil, the frequency response characteristic of the device is determined substantially wholly by the relative values of the impedance elements in the circuit in which the device Works, this circuit being hereinafter termed the work circuit. For example, the pick-up device may work directly into the grid of an amplifier tube 23, and across a suitable grid leak resistance 2|, as is shown in Figure 5. According to my invention this work circuit is arranged or tuned to have a natural resonant frequency within the middle vocal frequency range, as at a frequency between 1200 to 2000 cycles. Preferably, I employ high impedance pick-up coils having many turns of fine wire and control the winding space factor of the coils so that their effective internal distributed capacity plus the capacity of the shielded leads to the coils-which total capacity is referred to as 25 in Figure 5-will in combination with the inductance of the coils give the desired natural resonant frequency. Of course the distributed capacity 25 may be supplemented by a, lumped external capacity, if desired. In tuning the pick-up device, though, I preferably choose relative values of resistance to reactance which will obtain a very mild accentuation in response of the pick-up device at its resonant frequency. Typically, I find that an accentuation of from 3 to 6 dbs. at the resonant frequency over that obtained at one octave from the resonant frequency is satisfactory.

It will be noted that the pick-up device which I provide is very well adapted for picking up telephonic voice signal from hand combination telephone sets. For instance, the device may be conveniently inserted in the space 2| normally provided below the usual hand combination set 22 as is shown in Figure 1. As so placed, the pick-up device is very desirably out of view where it will not deface the appearance of the telephone instrument. Yet the pick-up device may be conveniently oriented for maximum coupling with the telephone induction coil merely by shifting it horizontally below the hand combination set.

While my invention has especial utility and is herein illustrated and described as a telephone pick-up device, it will also be understood that various changes and modifications may made in the embodiment herein disclosed without departing from the scope of my invention, which I endeavor to express according to the following claims.

I claim:

1. A magnetic pick-up device for response to the stray field of an induction coil, comprising a solenoidal type pick-up coil having a magnetic core, said core having a ratio of length to square root of cross-sectional area which is at least of the order of 15.

2. A magnetic pick-up device for response to the stray field of an induction coil, comprising a. solenoidal type pick-up coil having a magnetic core, said core having a ratio of length to square root of cross-sectional area which is at least of the order of 15.

3. A magnetic pick-up device selectively responsive to magnetic fields of different divergence, comprising a. pair of pick-up coils having magnetic cores, said coils being spacedly held in substantially parallel relationship and arranged magnetically opposing, and said coils being of the solenoidal type.

4. A magnetic pick-up device for response to the stray field of an induction coil and adapted to be substantially unresponsive to fields originating from sources relatively distant, comprising a pair of solenoidal type coils having magnetic cores, said coils being fixedly held in a substantially parallel relationship and arranged magnetically opposing, and the length of said coils being substantially twice the distance of spacing between the coils.

5. A magnetically selective pick-up device comprising a pair of magnetic core solenoidal type coils fixedly held in a substantially parallel relationship and connected series-opposing, said coils having a substantial spacing from one another relative to their lengths, and the ratio of the lengths of said coils to their outside diameters being at least a several times multiple of unity.

6. A magnetic pick-up device for response to the stray field of an induction coil and adapted to be substantially unresponsive to fields originating from sources relatively distant, comprising a pair of solenoidal type coils having magnetic cores and arranged magnetically opposing, the ratio of length to square root of cross section area of said coils being at least of the order of 15.

7. A magnetic pick-up device for response to the stray field of an induction coil and adapted to be substantially unresponsive to fields originating from sources relatively distant, comprising a pair of solenoidal type coils arranged magnetically opposing, said coils having magnetic cores provided with shoes at the core ends, said shoes extending radially of the coils at least to the peripheral surfaces of the coils.

8. A ma netic pick-up device for response to the stray field of an induction coil and adapted to be substantially unresponsive to fields originating from sources relatively distant, comprising a pair of solenoidal type coils spacedly held in substantially parallel relationship and arranged magnetically opposing, said coils having magnetic cores provided with shoes at the core ends, said shoes extending radially of the coils in a direc telephone system comprising a telephone box ineluding a telephone induction coll connected in the telephone system; a solenoidal-type pick-up coil mounted adjacent to said box and connected to the phonograph, said pick-up coil being oilset radially from said induction coil and having its axis in a plane substantially parallel to the axis of the induction coil, and said pick-up coil havme a ratio of length to outside diameter which is ill at least of the order of 5 JOSEPH C. ADAMS.

Certificate of Correction Patent No. 2,381,097.

August 7, 1945.

JOSEPH O. ADAMS It is hereby certified that errors appear in the printed specification oi the above numbered patent requiring correction as follows: Page 3, first column, hne 2 8, for 3% long read 3%" long; and second column, line 7, after the word may msert be; lines 21 to 23, strike out to square root of cross-sectional area which 1s at least of the order of 15 and insert instead to outside diameter which is at least 0j the order of 5; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 3rd day of September, A. D. 1946.

LESLIE FRAZER,

First Assistant Commissioner of Patents. 

